1. Field of the Invention
The present invention relates to a fuel injection system for an internal combustion engine. More particularly, the present invention relates to a fuel injection system in which injection valves have been provided on the upstream side and on the downstream side thereof, respectively, with a throttle valve interposed therebetween.
2. Description of Background Art
When the fuel injector is provided upstream from the throttle valve, the volumetric efficiency is improved because heat is taken from intake air when injection fuel vaporizes. Therefore, the engine output can be increased as compared with when the fuel injector is provided downstream from the throttle valve. On the other hand, when the fuel injector is provided on the upstream side, a distance between the fuel injection port of the upstream fuel injector and the combustion chamber inevitably increases. Accordingly, a response lag occurs in fuel transport as compared with when the fuel injector is provided downstream from the throttle valve. This causes the driveability of the engine to deteriorate.
In Japanese Patent Laid-Open Nos. 4-183949 and 10-196440, it has been attempted to solve such technical problems, to improve engine output and to ensure that driveability is compatible with the engine output. In the above documents, a fuel injection system has been disclosed in which fuel injectors have been provided on the upstream side and on the downstream side from the intake pipe, respectively, with the throttle valve interposed therebetween.
FIG. 7 is a cross-sectional view showing a major portion of an internal combustion engine according to the background art in which two fuel injectors have been arranged with a throttle valve 52 of an intake pipe 51 interposed therebetween. A downstream fuel injector 50a has been arranged on a side portion of the downstream side (engine side) of the throttle valve 52 and an upstream fuel injector 50b has been arranged on the upstream side (air cleaner side) of the throttle valve 52. A lower end portion of the intake pipe 51 is connected to an intake passage 52. An intake port 53 faces a combustion chamber of the intake passage 52 and is opened and closed by an intake valve 54.
The fuel injection quantity of each fuel injector is determined with plural parameters including the throttle opening as a function. However, volumetric efficiency within the combustion chamber is dependent on the intake temperature. Accordingly, an electronic controlled fuel injection system detects the intake temperature TA to control in such a manner that the injection quantity is relatively reduced as the intake temperature TA becomes higher.
The intake temperature TA is preferably detected immediately before the combustion chamber. However, when a temperature sensor is provided at the portion concerned, the intake efficiency of an air-fuel mixture into the combustion chamber is deteriorated. Accordingly, in an engine in which two fuel injectors are arranged, the temperature sensor is often provided on the upstream side from the fuel injection area of the upstream fuel injector 50b. 
However, the air within the intake pipe is cooled by the fuel injected from the upstream fuel injector 50b. Accordingly, a difference occurs between the intake temperature to be detected by the temperature sensor and the intake temperature immediately before the combustion chamber. This causes problems in detecting the correct intake temperature TA.